The present invention describes an improvement to and extension of the channel signal processing method and means described in copending application U.S. Ser. No. 07/270,895, filed Nov. 14, 1988 and in the improvement thereto as described in copending application U.S. Ser. No. 07/470,020, filed Jan. 25, 1990.
The first cited copending application describes a five-sample look-ahead algorithm for processing sample values in a (1,7) coded partial response channel, when the pulse shapes are ideal; i.e., when the read response to the positive and negative magnetic transitions are the pulses with the sample values . . . ,0,2,4,2,0, . . . and . . . ,0,-2,-4,-2,0, . . . , respectively. It also shows how to adjust the test thresholds of the algorithm in order to accommodate variations in the pulse shape. The thresholds may be optimized empirically in response to the boundary crossings in actual test data. Alternatively, the thresholds can be expressed as a function of the sample values . . . ,0,.alpha..sub.1,.beta..sub.1,.gamma..sub.1,0, . . . and . . . ,0,-.alpha..sub.2,-.beta..sub.2,-.gamma..sub.2, 0, . . . in the positive and negative readback pulses.
A five-sample look-ahead algorithm was used for processing channel sample values y.sub.0,y.sub.1,y.sub.2,y.sub.3,y.sub.4 to decode the current sample value y.sub.0. This algorithm works well when all error modes are caused by similar noise circumstances. However, in specific applications, such as magnetic tape and optical recording, when the readback pulse is substantially reduced in amplitude during a media defect or distorted towards being too wide, the available distance is reduced for specific pairs of sequences of sample values. This increases the risk of peak shift errors, and the reliability performance of the decoder, in such case, is predominantly determined by the lower margin of error associated with these, sequences.
The second above-cited prior application describes an enhancement to the processing method just described. It involves precomputing a peak-position check including expressions using up to six-sample values y.sub.0 -y.sub.5 ahead of the then current sample value y.sub.0. Use of this six- (instead of five) sample look-ahead algorithm provides increased signal detection reliability and increased tolerance to readback pulse distortion by substantially reducing the exposure due to peak shift patterns. However, when these peak shift errors are removed, the primary concern shifts to patterns which are more sensitive to missing or extra bit errors, particularly in the presence of defects and nonlinearities.
A phase check is required in both of the above-cited signal processing methods in order to limit the propagation of detection errors.
There is a need for a signal processing method wherein this sensitivity to missing or extra bit errors can be minimized and wherein full advantage of a (1,7) run-length-limited code constraint can be achieved and the phase check can be eliminated.